European leaders have called for an immediate international response after Belarus forced a Ryanair flight bound for Lithuania to land in Minsk on Sunday and arrested one of its passengers, a top opposition activist.

State media said online activist Roman Protasevich, resident in Lithuania, was detained in the Belarusian capital after Ryanair flight FR4978 from Athens to Vilnius was unexpectedly diverted to Minsk shortly before leaving Belarusian airspace.

European Commission president Ursula von der Leyen said that the forced landing was “utterly unacceptable” and called on Belarus to let all passengers travel safely to Vilnius. “Any violation of international air transport rules must bear consequences,” she wrote on Twitter.

Nato secretary-general Jens Stoltenberg tweeted: “This is a serious and dangerous incident, which requires international investigation. The Belarusian authorities must ensure the safe return of the crew and all passengers to Vilnius.”

Dominic Raab, British foreign secretary, said the UK was “alarmed” at reports of the forced landing and Protasevich’s arrest, adding: “We are co-ordinating with our allies. This outlandish action by Lukashenko will have serious implications.”

Lithuania’s president Gitanas Nauseda called for the activist’s swift release and said he would raise the matter at an EU summit on Monday. “I call on Nato and EU allies to immediately react to the threat posed to international civil aviation by the Belarus regime,” he said in a statement.

The Lithuanian foreign minister, Gabrielius Landsbergis, said he had spoken to Josep Borrell, the EU’s high representative for foreign affairs, describing the incident as an affront to the whole EU that must be addressed in the strongest possible terms.

Polish prime minister Mateusz Morawiecki wrote on Twitter that he would call for “immediate sanctions” against Belarus. “Hijacking of a civilian plane is an unprecedented act of state terrorism. It cannot go unpunished,” Morawiecki wrote.

Germany’s foreign minister, Heiko Maas, said: “Such an act cannot stand without clear reaction from the European Union.

“The fact that a flight between two EU countries was forced to stop under the pretext of a bomb threat is a serious interference with civil air traffic in Europe. We are very concerned about reports that journalist Roman Protasevich was arrested in this way,” he said. “The upcoming informal European Council should . . . address the incident.”

Protasevich, 26, is the former editor of Nexta, the Warsaw-based media group that played a prominent role in both covering and directing huge protests that erupted against Lukashenko last year after he claimed victory in a deeply flawed election.

“From now — no one flying over Belarus — can be secure,” Sviatlana Tsikhanouskaya, Belarus’s exiled opposition leader, wrote on Twitter.

Belarusian president Alexander Lukashenko personally gave an “irrevocable command to turn the plane around and land it” before it left Belarusian airspace, according to a post on a semi-official presidential channel on messaging app Telegram.

In November, Belarus placed Protasevich on a terrorist watchlist and charged him with three protest-related crimes, the most serious of which carries a sentence of up to 15 years in prison.

Ryanair said the flight crew were “notified by Belarus [air traffic control] of a potential security threat on board” and instructed to divert to Minsk.

“Nothing untoward was found and authorities cleared the aircraft to depart together with passengers and crew after approximately five hours on the ground in Minsk,” it said in a statement. The company has “notified the relevant national and European safety and security agencies”.

According to messages sent by Protasevich to his colleagues, he noticed he was being followed by a man he suspected was a Belarusian KGB agent while at the Athens departure lounge. The man stood behind him in the boarding queue and tried to take a photo of his documents, the activist wrote in text messages to his colleagues. Protasevich said the man then asked him a “stupid question” in Russian and left.

The plane turned round near the Lithuanian border and landed in Minsk, according to flight tracking data. Andrei Gurtsevich, a senior air force commander, said Belarus had decided to scramble a MiG-29 fighter jet to accompany the plane after learning of a bomb threat”, according to Belta, the state news agency. Airport officials later said the bomb threat was “false”.

Belarusian state television said officials did not know Protasevich was on the flight, claiming that he remained in the airport undetected until his girlfriend sent a photo of him to another dissident blogger.

But a fellow passenger told Lithuanian news site Delfi that Protasevich began panicking when he realised the plane was heading for Minsk, holding his head in his hands.

Once the plane landed, officials immediately detained Protasevich, who was visibly trembling, and seized his luggage, according to the unnamed passenger.

“We asked him what was happening. He said who he was and added: ‘They’ll execute me here,’” Delfi quoted the passenger as saying.

Nexta has provoked Lukashenko’s ire for its publication of leaks from official sources, as well as for its coverage of the brutal crackdown that Lukashenko’s regime unleashed in response to last year’s protests. Its channel on Telegram has more than 1.2m subscribers, a huge audience in a country of just 9.5m people.

Hundreds of thousands of Belarusians took to the streets in an unprecedented show of discontent after the former collective farm boss, who has ruled Belarus with an iron fist for 27 years, claimed victory over Tsikhanouskaya in the presidential election last year.

Most of the opposition’s main figures have since either been forced into exile, such as Tsikhanouskaya, who is based in Lithuania, or imprisoned.